The present invention relates to a waveguide for an optical circuit, and a method of fabrication thereof.
The method relates in particular to the fabrication of a waveguide for an optical circuit with smoothed waveguide core boundaries. More specifically, the method relates to the fabrication of a rounded, elliptical or circular waveguide core by the isotropic diffusion of dopants in a core layer of a phosphosilicate waveguide wafer, such that the diffused core layer forms the circular waveguide core. In this manner, a core may be formed which is symmetric about the core axis.
This diffusion is thermally promoted either during the deposition of an uppercladding layer or by subsequent thermal processing of the waveguide wafer.
The general process of fabricating a glass waveguide for optical circuits comprises forming at least one buffer layer, e.g. a thermal oxide layer, on a silicon wafer substrate. Additional buffer layers and/or at least one lower cladding layers may then be formed on top of the buffer layer. A core layer composed of a doped silica film is then formed on top of the buffer layer or lower cladding layer.
The core layer is then etched, for example, by reactive ion techniques, to form a square or rectangular waveguide or other suitable cross-sectional profile. The etched core is then embedded by an upper cladding layer. The core layer refractive index is usually higher than that of the surrounding layers. This concentrates the propagation of light in the core layer.
Planar channel waveguides are usually formed using dry etch methods to produce waveguides with square or rectangular cross-sections. Such angular waveguides have several disadvantages, in particular the geometrical mismatch between the waveguides and optical fibres in an optical circuit. The production of channel waveguides with a circular cross-section is particularly advantageous in that this increases the transmission efficiency between the waveguide and the rest of an optical circuit.
Channel waveguides are also susceptible to scatter loss (Mie scattering) due to imperfections in their sidewalls. This is reduced by smoothing the profile of the waveguide and this provides low propagation loss in the waveguides.
Circular optical waveguides are known in principle (for example, see Sun et al., xe2x80x9cSilica-based circular cross-sectioned channel waveguidesxe2x80x9d, IEEE Photonics Technology Letters, 3, p.p. 238-240, 1991). Sun et al., disclose large dimension (xcx9c50 xcexcm) GeO2 doped silica waveguides which are reactive ion etched to form rectangular channel cross-sections. This method involves depositing a lower cladding layer with a reduced amount of Germanium doped silicon on to the wafer substrate prior to the deposition of a core layer. When the wafer is placed in the selective wet etch, the lower cladding layer is etched at a much faster rate to form a pedestal underneath the core region.
According to Sun et al., the waveguide can then be heated above the core softening temperature so that the surface tension of the glass functions to round the waveguide core. Such wet etching techniques are time consuming and moreover, do not offer truly circular cross sections as the core cannot be rounded at the interface between the core layer and the pedestal (i.e., the lower cladding layer lying directly beneath the core).
The current invention in contrast relies on the mobility of dopant ions in a square or rectangular etched core to migrate outwards into both upper and lower cladding layers. This forms waveguides which have substantially smoothed boundary walls, in particular the side walls are smoothed.
Further diffusion rounds the core region, and providing the diffusion is sufficiently isotropic the core region becomes sufficiently rounded to form a circular waveguide. This diffusion is thermally promoted either during the consolidation of the upper cladding layer or during subsequent thermal processing. By selecting the composition of the upper and lower cladding layers, the refractive indexes and consolidation temperatures can be chosen to modify the rate at which the core dopant ions diffuse into each layer and the elipticity of the resulting waveguide core accordingly adjusted.
According to a first aspect of the present invention, there is provided a waveguide for an optical circuit comprising:
a substrate;
a doped lower cladding layer;
a doped waveguide core formed on the lower cladding layer; and
a doped upper cladding layer embedding the waveguide core;
wherein the waveguide core includes mobile dopant ions which have diffused into the upper cladding layer and the lower cladding layer to form an ion diffusion region around said waveguide core such that the waveguide core boundary walls are substantially smooth.
Preferably, the waveguide further includes a buffer layer formed on the substrate and wherein the lower cladding layer is formed on the buffer layer. The substrate may comprise silicon and/or silica and/or sapphire. The buffer layer may include a thermally oxidised layer of the substrate.
Preferably, the buffer layer comprises doped silica.
Preferably, the thickness of the buffer layer is in the range 0.2 xcexcm to 20 xcexcm.
The lower cladding layer may comprise doped silica. The lower cladding layer may include at least one Phosphorus oxide and/or at least one Boron oxide.
Preferably, the lower cladding layer includes at least one Phosphorus oxide and at least one Boron oxide, wherein the Phosphorus oxide to Boron oxide ratio is such that the lower cladding layer refractive index is substantially equal to the refractive index of the buffer layer.
The lower cladding layer may include doped silica, at least one Phosphorus oxide and at least one Boron oxide, wherein the silica:Phosphorus oxide:Boron oxide ratio is in the range of 75 to 95 wt % silica:1 to 7 wt % Phosphorus oxide:4 to 18 wt % Boron oxide.
Preferably, the lower cladding layer has a silica:Phosphorus oxide:Boron oxide ratio in the range of 80 to 90 wt % silica:2.5 to 6 wt % Phosphorus oxide:7.5 to 14 wt % Boron oxide.
More preferably, the lower cladding layer has a silica; to Phosphorus oxide; to Boron oxide ratio of 82 wt % silica; to 5 wt % Phosphorus oxide; to 13 wt % Boron oxide.
Preferably, the thickness of the lower cladding layer is 1 xcexcm to 20 xcexcm.
The waveguide core may comprise doped silica. The mobile dopant ions of the waveguide core may include Phosphorus and/or Fluorine and/or compounds of these elements. Dopant ions of the waveguide core may include Phosphorus and/or Fluorine and/or Aluminium and/or Boron and/or Germanium and/or Tin and/or Titanium and/or compounds of these elements.
Preferably, the waveguide core includes Phosphorus oxide and/or Boron oxide. More preferably, the waveguide core comprises P2O5xe2x80x94SiO2.
Preferably, the refractive index of the waveguide core differs from that of the lower cladding layer by at least 0.05%.
Preferably, the waveguide core includes silica, and at least one Phosphorus oxide, wherein the silica to Phosphorus oxide ratio is in the range of 75 to 95 wt % silica to 5 to 25 wt % Phosphorus oxide.
More preferably, the waveguide core has a silica to Phosphorus oxide ratio of 80 wt % silica to 20 wt % Phosphorus oxide.
Preferably, the thickness of the waveguide core is in the range 2 xcexcm to 60 xcexcm.
More preferably, the thickness of the waveguide core is 6 xcexcm.
Preferably, the lower cladding layer and the upper cladding layer refractive indices are substantially equal. The lower cladding layer and the upper cladding layer may comprise the same material.
Preferably, the waveguide core has a mobile ion dopant concentration higher than the mobile ion dopant concentration of the lower cladding layer or the upper cladding layer.
Preferably, the ion diffusion region is isotropic with respect to the waveguide core.
Preferably, the ion diffusion region surrounding the waveguide core forms a substantially rounded waveguide core.
More preferably, the rounded waveguide core is elliptical or circular in cross-section.
According to a second aspect of the invention, there is provided a method of fabricating a waveguide comprising the steps of: providing a substrate; forming a doped lower cladding layer; forming a doped core layer on the lower cladding layer; forming a waveguide core from the core layer; forming a doped upper cladding layer to embed the waveguide core; wherein mobile ion dopants included in the core layer undergo diffusion into the surrounding upper cladding layer and lower cladding layer to form an ion diffusion region around the waveguide core such that the waveguide core boundary walls are substantially smooth.
The method may include the step of forming a buffer layer on the substrate. The lower cladding layer may be formed on said buffer layer. The steps of forming each of the lower cladding layer, the core layer and the upper cladding layer may comprise the steps of: depositing each layer; and at least partially consolidating each layer.
Preferably any of the lower cladding layer, the core layer and the upper cladding layer partially consolidated after deposition is fully consolidated with the full consolidation of any other of the lower cladding layer, the core layer or the upper cladding layer.
Preferably, the diffusion of mobile ion dopants in the core layer occurs during the consolidation of the lower cladding layer and/or the upper cladding layer.
The method may further comprise at least one thermal processing step after the formation of the upper cladding layer, wherein during said thermal processing of the waveguide the mobile ion dopants in the core layer undergo diffusion into the surrounding layers. The substrate may comprise silicon and/or silica and/or sapphire. The buffer layer may include a thermally oxidised layer of the substrate. The buffer layer may comprise doped silica.
Preferably, the thickness of the buffer layer formed is in the range of 0.2 xcexcm to 20 xcexcm. The lower cladding layer may comprise doped silica. The lower cladding layer may include at least one Phosphorus oxide and/or Boron oxide. The lower cladding layer may include at least one Phosphorus oxide and at least one Boron oxide, wherein the Phosphorus oxide to Boron oxide ratio is such that the lower cladding layer refractive index is substantially equal to the refractive index of the buffer layer.
Preferably, the lower cladding layer includes silica, at least one Phosphorus oxide and at least one Boron oxide, wherein the silica; to Phosphorus oxide; to Boron oxide ratio in the range of 75 to 95 wt % silica; to 1 to 7 wt % Phosphorus oxide; to 4 to 18 wt % Boron oxide.
Preferably, the lower cladding layer has a silica; to Phosphorus oxide; to Boron oxide ratio in the range of 80 to 90 wt % silica; to 2.5 to 6 wt % Phosphorus oxide; to 7.5 to 14 wt % Boron oxide.
More preferably, the lower cladding layer has a silica; to Phosphorus oxide; to Boron oxide ratio of 82 wt % silica; to 5 wt % Phosphorus oxide; to 13 wt % Boron oxide.
Preferably, the thickness of the lower cladding layer is 1 xcexcm to 20 xcexcm.
Preferably, the core layer comprises doped silica. The mobile dopant ions of the waveguide core may include Phosphorus and/or Fluorine and/or compounds of these elements. The dopant ions of the waveguide core may include Phosphorus and/or Fluorine and/or Aluminium and/or Boron and/or Germanium and/or Tin and/or Titanium and/or compounds of these elements.
The core layer may include Phosphorus oxide and/or Boron oxide.
Preferably, the core layer comprises P2O5xe2x80x94SiO2.
Preferably, the refractive index of the waveguide core differs from that of the lower cladding layer by at least 0.05%.
Preferably, the waveguide core includes silica and at least one Phosphorus oxide, wherein the silica to Phosphorus oxide ratio is in the range of 75 to 95 wt % silica to 5 to 25 wt % Phosphorus oxide.
More preferably the waveguide core has a silica to Phosphorus oxide ratio of 80 wt % silica to 20 wt % Phosphorus oxide.
Preferably, the thickness of the waveguide core is in the range 2 xcexcm to 60 xcexcm.
More preferably, the thickness of the waveguide core is 6 xcexcm.
Preferably, the lower cladding layer and said buffer layer are formed substantially in the same step.
Preferably, the consolidation of the lower cladding layer is at a temperature or temperatures in the range 950xc2x0 C. to 1400xc2x0 C.
Preferably, the consolidation of the lower cladding layer is at a temperature or temperatures in the range 1100xc2x0 C. to 1350xc2x0 C.
Preferably, the consolidation of the core layer is at a temperature or temperatures in the range 950xc2x0 C. to 1400xc2x0 C.
More preferably, the consolidation of the core layer is at a temperature or temperatures in the range 1100xc2x0 C. to 1385xc2x0 C.
Preferably, the consolidation of the upper cladding layer is at a temperature or temperatures in the range 950xc2x0 C. to 1400xc2x0 C.
More preferably, the consolidation of the upper cladding layer is at a temperature or temperatures in the range 1100xc2x0 C. to 1350xc2x0 C.
The temperature or temperature range at which the lower cladding layer is consolidated may be greater than the temperature or temperature range at which the core is consolidated. The temperature or temperature range at which the upper cladding layer is consolidated may be substantially equal to the temperature or temperature range at which the core layer is consolidated.
At least one of the lower cladding layer, the core layer, and the upper cladding layer may be deposited by a Flame Hydrolysis Deposition process and/or Chemical Vapour Deposition process. The Chemical Vapour Deposition process may be a Low Pressure Chemical Vapour Deposition process or a Plasma Enhanced Chemical Vapour Deposition process.
Preferably, the consolidation is by fusing using a Flame Hydrolysis Deposition burner. Alternatively, the consolidation may be by fusing in a furnace.
Preferably, the step of fusing the lower cladding layer and the step of fusing the core layer are performed simultaneously.
Preferably, the waveguide core formed from the core layer is square or rectangular in cross-section.
The waveguide core may be formed from the core layer using a dry etching technique and/or a photolithographic technique and/or a mechanical sawing process.
The dry etching technique may comprise a reactive ion etching process and/or a plasma etching process and/or an ion milling process.
Preferably, the diffusion of the said mobile dopant ions from the waveguide core is isotropic.
Preferably, the diffusion of the said mobile dopant ions from the waveguide core swells the boundary walls of the waveguide core.
More preferably, diffusion of the said mobile dopant ions swells the boundary walls of the waveguide core to form a substantially rounded waveguide core.
The rounded waveguide core formed may be elliptical or circular in cross-section.
The smoothing of the walls reduces scattering losses and lowers the propagation losses for the waveguides. Coupling losses between optical circuits and optical fibre are also reduced due to the improved geometry of the waveguide core. For example, the enhanced roundedness of the core of the waveguide enables it to be coupled more efficiently to optical fibre which has an appropriate circular or elliptical cross-section.